The substructure of tight junctions is investigated by direct freezing techniques that avoid any chemical fixation and serve to increase the resolution of individual membrane components. The backbone of the tight junction of each of the paired component membranes is a continuous cylinder. This model replaces the previous view that tight junctions are comprised of rows of intramembrane proteins; the rod-shaped structures are now interpreted as inverted cylindrical micelles of membrane lipids. Recent evidence is that a similar model is applicable to tight junctions in invertebrates. Evidence for this model is also being gathered from investigations of pure lipid bilayer systems which are induced to form non-planar micellar phases by addition of calcium ion. Cylindrical micelles identical to those seen at tight junctions are found embedded in these lipid bilayers. Assembly of tight junctions is being studied in cultured anphybian epithelial cells which form continuous cell monolayers. Ca++ removal from the culture medium results in rapid disruption of the monolayer structure and disassembly of tight junctions which break down into small single cylindrical segments in the interior of the membrane of each separate cell. Cell polarity and cytoskeletal changes accompanying tight junction formation and disassembly are being followed by video enhanced differential interference microscopy and immunofluorescence. The true inner surfaces of naturally occurring tight junctions are being visualized by deep-etching. A filamentous structure on the surface of this membrane, which is coextensive with the cylindrical micelle, may account for the protein associated with tight junctions, and may explain how cylindrical micelles are stabilized in certain regions of the cell membrane. How tight junctions serve in the blood-brain barrier system to prevent small charged solutes from entering the brain is made clear by this new model of tight junction structure.